Fluid Dynamics

Omniverse™ Flow is an Eulerian fluid simulation for smoke and fire, leveraging a sparse voxel grid for unbounded simulation domain.

Using Omniverse™ Flow

Getting Started

To start using Omniverse™ Flow, the omni.flowusd extension must be enabled. Make sure omni.flowusd is enabled in the Window->Extensions window.

Omniverse™ Flow must also be enabled in the RTX renderer. It is enabled automatically if Flow objects are created from the menu.

To manually enable Flow in Common Render Settings:

Presets

Presets are the fastest way to get a working Flow effect. They can be created through Create->Flow->Preset.

Press Play to begin simulation.

Create->Flow->Preset->Set Postprocessing is also useful to quickly enable bloom to make fire appear bright.

Transforms

To move Flow emitters easily, make the Flow emitter prim a child of an XForm.

Render Settings

For more light interaction, the following render settings can be enabled. Can also enable bloom and indirect diffuse GI.

Manual Setup

A working simulation can also be achieved by adding the necessary Flow prims to a stage. Simulate/Offscreen/Render Settings are required for any simulation. Be sure to enable Flow in the Common Render Settings. (Presets do this automatically.)

Press Play to begin simulation.

Layers

Flow layers provide a mechanism to allow multiple effects at the same time, sharing the same memory pool. UI is currently low level, with layers being specified by a simple integer. For a simulation to behave properly, an emitter and simulate/offscreen/render settings must all share a common layer. Flow presets automatically assign a unique layer, to prevent collisions with other presets. The UI for layers will be improved with future releases.

As an example, create a fire preset as specified above. Now Create->Flow->Add Box Emitter. Delete the flowEmitterSphere in the stage.

At this point, no simulation will occur. Select flowSimulate in the stage to inspect its layer assignment.

Select flowEmitterBox and set its layer attribute to the same value. Visual simulation should now occur.

Colormap

The omni.flowusd also includes a simple colormap editor to change the mapping of temperature to color. Create->Flow->Open Colormap Editor will open the colormap editor window. For the fire preset above, selecting flowOffscreen/colormap in the stage will populate the colormap editor.

Note for fire, colormap values can far exceed 1.0. This is needed for properly interaction with bloom. Once values are adjusted, hit Apply Flow Color to see changes applied.

Understanding Omniverse™ Flow

Cell Size

Cell Size has one of the most significant impacts on both appearance and performance. Keep in mind cutting cell size in half is an 8x increase in cells and as a result simulation time.

Channels

Flow currently has standard channels. The four density channels are temperature, fuel, burn, smoke, packed in that order. The four velocity channels are velocity_x, velocity_y, velocity_z, divergence, packed in that order. Advection and emitters expose independent parameters for each of those channels.

Couple Rate

Couple rate is used on Flow emitters. It specifices a rate of correction between an existing grid value and a target emitter value. Higher values like 120 can take the grid instantly to the emitter target values. High couple rates can be useful with fast moving emitters. Lower couple rates are very useful for slow moving emitters. By making the move towards the target value lazy, simulation can actively occur inside the emitter. This typically results in less artifacts and a more natural transition from inside to outside the emitter.

Effect Lifetime

The damping/fade on advection largely controls effect lifetime. Damping is proportional to the current grid value. Fade is a fixed units / second rate.

Effect Scale

Vorticity forceScale has a significant impact on perceived scale. Higher forceScale tends to make effects appear larger.

Combustion Modeling

Default today includes a simple combustion model. Settings like buoyancyPerBurn help create mushroom cloud motion expected from explosions. Parameters like burnPerTemp and fuelPerBurn can also affect how fast combustion occurs. divergencePerBurn also controls expansion due to combustion. Higher values are useful for explosions.

Velocity

With velocity, more is not always more. This simulation is based on stable fluids, where high velocities don’t necessarily behave as expected, but the simulation does not explode. Coarser cell size will effectively behave better will higher world scale velocities. Support for velocity substepping and overall shorter simulation steps are exposed, that improve high velocity behavior with a performance cost.

Rendering

Rendering today uses the Omniverse™ Flow reference implementation. Future versions will likely lean on native volume support in the RTX renderer. Basic Omniverse™ Flow rendering provides a colormap to map temperature to color/alpha for fire/smoke. Also included is self shadowing.

Collision

Collision support today is very limited. However emitters can be adapted for this purpose. Set allocationScale to 0.0, since collision only needs to be active when the simulation is visibly active. Set temperature/fuel/burn/smoke/divergence couple rates to zero, as we only need to control velocity. Set target velocity to (0, 0, 0). Set physicsVelocityScale to 1.0 for moving objects, to include body velocity. Set coupleRateVelocity to 120 or higher, to drive velocity to body velocity within a single frame. Set applyPostPressure to true, as this guarantees velocity at the end of the frame will match body velocity.

Settings

Simulate Settings

Property	Result
blockMinLifetime	Minimum lifetime of any allocate block. Avoids premature deactivation caused by GPU readback delay.
densityCellSize	Size of density cells in world units. By default velocity cell size is twice density cell size.
enableLowPrecisionDensity	Use 8-bit channels for temperature, fuel, burn, smoke
enableLowPrecisionVelocity	Use 8-bit channels for velocity, divergence
enableSmallBlocks	Use 16x8x8 density cell blocks instead of the default 32x16x16
enableVariableTimeStep	Take step every frame. Can cause inconsistent simulation behavior for highly variable time step values, but useful when frame time is fixed already.
forceClear	Kills all active block allocations in this layer
forceDisableCoreSimulation	Disable most simulation components. Emitters still function.
forceDisableEmitters	Disable all emitters in this layer
forceSimulate	Force simulate, even if timeline is paused.
interpolateTimeSteps	Useful for motion blur in movie capture. Produces sub-frames between full simulation steps.
layer	See Layer section above.
maxStepsPerSimulate	Limit on steps per frame to avoid excessive steps when application is running slow.
simulateWhenPaused	Run simulation even if deltaTime is 0.0.
stepsPerSecond	Simulation update rate
timeScale	Scales input time. Useful for slomo.
velocitySubSteps	Higher values improve behavior with high speed effects, at the cost of performance.
advection	See Advection section.
vorticity	See Vorticity section.
pressure	See Pressure section.
summaryAllocate	See SummaryAllocate section.

Advection

Property	Result
buoyancyMaxSmoke	Smoke clamp value applied before computing smoke buoyancy, default 1.0
buoyancyPerSmoke	Buoyant force per unit smoke, default 0.0
buoyancyPerTemp	Upward force relative to temperature. Higher values help produce mushroom effects.
burnPerTemp	Rate of burn relative to temperature. Higher burn values affect fuel consumption and divergence (expansion) terms.
combustionEnabled	If false, only buoyancyMaxSmoke and buoyancyPerSmoke are active
coolingRate	Temperature fade, applied on a per cell level. (Not conduction/convection.)
divergencePerBurn	Expansion relative to burn amount. Units comparable to velocity in terms of scale. Higher values produce visible expansion, but can remove detail.
downsampleEnabled	Boolean to enable/disable downsample. Downsample needed for SummaryAllocate
enabled	Boolean to enable/disable advection.
forceFadeEnabled	If true, fade will be applied even if advection is disabled
fuelPerBurn	Fuel consumed relative to burn amount. Lower values will make a given amount of fuel last longer.
globalFetch	Improves advection quality for high velocities, with some increase in runtime cost. (Can remove some banding artifacts)
gravity	Used to determine the ‘up’ direction for buoyancy.
ignitionTemp	Minimum temperature for combustion.
smokePerBurn	Smoke increase relative to burn amount.
tempPerBurn	Temperature increase relative to burn amount.
smoke	See Advection Channel section.
velocity	See Advection Channel section.
divergence	See Advection Channel section.
temperature	See Advection Channel section.
fuel	See Advection Channel section.
burn	See Advection Channel section.

Advection Channel

Property	Result
damping	Reduce channel proportional to current value. Valid range is 0 to 1, where 1 takes the value to 0 immediately.
fade	Reduce channel at a constant rate per second. Valid range is 0 to infinity effectively.
secondOrderBlendFactor	Second order correction factor. Value range is 0.0 to 1.0. 0.5 is generally a safe default. Higher values increase the sharpness, but comes with potentially banding artifacts. Use extra caution with velocity, where effects are even more pronounced.
secondOrderBlendThreshold	Minimum value to apply second order correction. Reduces simulation cost for cells near zero, with minimal/zero visual artifacts.

Vorticity

Property	Result
burnMask	Allows force to be applied relative to burn value in grid.
constantMask	Allows force to be applied without respect to local velocity.
densityMask	Allows force to be applied relative to divergence (Parameter name should be fixed.)
enabled	Boolean to enable/disable vorticity confinement.
forceScale	Control intensity of effect
fuelMask	Allows force to be applied relative to fuel value in grid.
smokeMask	Allows force to be applied relative to smoke value in grid.
temperatureMask	Allows force to be applied relative to temperature value in grid.
velocityLinearMask	Makes force directly proportional to local velocity Math: velocityLinearMask * speed
velocityLogScale	This primarily exists to allow rescaling of Omniverse™ Flow effects to different coordinate systems.
velocityMask	Velocity log mask, works together with velocityLogScale. Makes local linear velocity a requirement for forces to be applied. The log part makes the force applied with respect to velocity a diminishing return to avoid instability.

Pressure

Property	Result
enabled	Boolean to enable/disable the pressure solver.

SummaryAllocate

Property	Result
enableNeighborAllocation	If false, SummaryAllocate preserves active blocks, but does not attempt allocation of new blocks.
smokeThreshold	Minimum smoke to keep a block active.
speedThreshold	Minimum speed to keep a block active, given speedThresholdMinSmoke is satisfied.
speedThresholdMinSmoke	Minimum smoke value for speedThreshold to apply.

Offscreen Settings

Property	Result
layer	Layer ID to apply these settings to
colormap	See Colormap section.
shadow	See Shadow section.
debugVolume	See DebugVolume section.

Colormap

Property	Result
colorScale	Multiplier applied to RGB components
resolution	Number of texels in colormap
rgabPoints	RGBA values of control points, RGB can be over 1.0 for HDR effects.
xPoints	X coordinates of control points, maps to temperature. Valid over 0 to 1

Shadow

Property	Result
attenuation	Rate of light blockage. Higher values increase self shadow effect.
coarsePropagate	Compute shadows at 2x2x2 coarser grid. Provides good quality at a much lower runtime cost.
enabled	Boolean to enable/disable self shadowing.
isPointLight	If true, point light, else directional light.
lightDirection	Directional light. Points to light.
lightPosition	Point light position.
minIntensity	Provide ambient level
numSteps	Number of steps per self shadow ray, divide by 2 when coarsePropagate is enabled.
stepOffsetScale	Ray march offset to avoid a cell shadowing itself.
stepSizeScale	Ray march step size relative to cell size. Higher values increase quality and performance cost.

DebugVolume

Property	Result
enableSpeedAsTemperature	Write speed value to temperature channel, to visualize in ray march.
enableVelocityAsDensity	Write abs() of velocity channels to density xyz to visualize in ray march.
velocityScale	Scale applied to velocity to control brightness for ray march result.

Render Settings

Property	Result
layer	Layer ID to apply these settings to
rayMarch	See RayMarch section

RayMarch

Property	Result
attenuation	Rate of light blockage. Higher values make the volume look more opaque
colorScale	Multiplier applied to RGB values during ray march
enableBlockWireframe	Visualizes active blocks for debug purposes
enableRawMode	If true, do not apply colormap, pass raw density xyzw as color
shadowFactor	The degree to which the z shadow channel scales the RGB channels from the colormap. 1.0 is default. 0.0 disables self shadowing effectively.
stepSizeScale	Ray march step size relative to cell size. Higher values increase quality and performance cost.
cloud	See RayMarchCloud section.

RayMarchCloud

Property	Result
ambientColor	Color in shadowed regions.
ambientMultiplier	Scale to adjust brightness of ambient color.
attenuationMultiplier	Controls how fast cloud shadows go dark.
densityMultiplier	Scales input density.
enableCloudMode	If true, default ray march is disabled, these settings are used instead.
numShadowSteps	More steps can improve quality, but add performance cost.
shadowStepMultiplier	Larger steps cover more of world space, at risk of artifacts.
sunDirection	Direction to sun.
volumeBaseColor	Base color for scattering.
volumeColorMultiplier	Scale to adjust brightness of volume base color.

Emitter Sphere

Sphere emitters are the simplest emitter type. Defined by a position and radius.

Property	Result
allocationScale	1.0 allocates blocks needed to contain sphere. 0.0 disables allocation. Values higher than 1.0 can be used to add additional padding.
applyPostPressure	If true, apply velocity after pressure solve. This is useful for collision, as it allows hard enforcement of velocity inside moving bodies.
burn	Target burn value.
coupleRateBurn	Couple rate for burn channel.
coupleRateDivergence	Couple rate for divergence channel.
coupleRateFuel	Couple rate for the fuel channel.
coupleRateSmoke	Couple rate for the smoke channel.
coupleRateTemperature	Couple rate for the temperature channel.
coupleRateVelocity	Couple rate for the velocity channel.
divergence	Target divergence value.
enabled	Boolean to enable/disable emitter
fuel	Target fuel value.
layer	Layer ID of simulation this emitter effects
multisample	Test sphere overlap at 8 locations instead of 1
numSubSteps	Substeps per simulation update. More steps produce a smoother result with moving emitters.
physicsVelocityScale	If set to 1.0, the body velocity will be added to the target velocity. This is useful for collision effects, where the body velocity and grid cell velocity should align.
position	Position of sphere
radius	World space radius of sphere
radiusIsWorldSpace	If true, do not apply xform scale to radius
smoke	Target smoke value.
temperature	Target temperature value.
velocity	Target velocity value.
velocityIsWorldSpace	If true, do not rotate velocity by xform

Emitter Box

Box emitters support emission from an oriented box. Clipping planes can also be added to support convex hulls.

Property	Result
allocationScale	1.0 allocates blocks needed to contain box. 0.0 disables allocation. Values higher than 1.0 can be used to add additional padding.
applyPostPressure	If true, apply velocity after pressure solve. This is useful for collision, as it allows hard enforcement of velocity inside moving bodies.
burn	Target burn value.
clippingPlanes	List of clipping planes in local space.
coupleRateBurn	Couple rate for burn channel.
coupleRateDivergence	Couple rate for divergence channel.
coupleRateFuel	Couple rate for the fuel channel.
coupleRateSmoke	Couple rate for the smoke channel.
coupleRateTemperature	Couple rate for the temperature channel.
coupleRateVelocity	Couple rate for the velocity channel.
divergence	Target divergence value.
enabled	Boolean to enable/disable emitter
fuel	Target fuel value.
halfSize	Half of box width/height/depth in local units
layer	Layer integer. See Layer section.
multisample	Test sphere overlap at 8 locations instead of 1
physicsVelocityScale	If set to 1.0, the body velocity will be added to the target velocity. This is useful for collision effects, where the body velocity and grid cell velocity should align.
smoke	Target smoke value.
temperature	Target temperature value.
velocity	Target velocity value.
velocityIsWorldSpace	If true, do not rotate velocity by xform

Emitter Point

Point emitters allow simple arrays to define points that are splatted to the simulation.

Property	Result
allocateMask	Default allocation behavior. If true and allocateMasks is empty, all points will trigger allocation.
applyPostPressure	If true, apply velocity after pressure solve. This is useful for collision, as it allows hard enforcement of velocity inside moving bodies.
burn	Default target burn value.
burnScale	Multiplier for burn and pointBurns.
colorIsSrgb	If true, perform color correction on temperature, fuel, burn channels
coupleRateBurn	Default couple rate for burn channel.
coupleRateDivergence	Default couple rate for divergence channel.
coupleRateFuel	Default couple rate for fuel channel.
coupleRateSmoke	Default couple rate for smoke channel.
coupleRateTemperature	Default couple rate for temperature channel.
coupleRateVelocity	Default couple rate for velocity channel.
divergence	Default target divergence value.
divergenceScale	Multiplier for divergence and pointDivergences.
enabled	Boolean to enable/disable emitter
enableStreaming	If true, only apply streamingBatchSize per frame
fuel	Default target fuel value.
fuelScale	Multiplier for fuel and pointFuels
layer	Layer integer. See Layer section.
numSubSteps	If numSubSteps > 1, Interpolates point position using velocity
pointAllocateMasks	Optional array of allocate settings per point.
pointBurns	Array of target burn values for each position.
pointColors	Array of temperature, fuel, burn values, used for color with ray march in raw mode.
pointCoupleRateBurns	Array of burn couple rates for each position.
pointCoupleRateDivergences	Array of divergence couple rates for each position.
pointCoupleRateFuels	Array of fuel couple rate for each position.
pointCoupleRateSmokes	Array of smoke couple rates for each position.
pointCoupleRateTemperatures	Array of temperature couple rates for each position.
pointCoupleRateVelocities	Array of velocity couple rates for each position.
pointDivergences	Array of target divergence values for each position.
pointFuels	Array of target fuel values for each position
pointPositions	Array of local space positions. Determines total point count.
pointSmokes	Array of target smoke values for each position
pointsPrim	Optional relationship to UsdGeomPoint, to source arrays
pointTemperatures	Array of target temperature values for each position.
pointVelocities	Array of target velocity values for each position.
smoke	Default target smoke value.
smokeScale	Multiplier for smoke and pointSmokes
streamClearAtStart	If true, clears entire layer when streaming start
streamOnce	If true, apply each point to the grid only once
temperature	Default target temperature value.
temperatureScale	Multiplier for temperature and pointTemperatures
updateCoarseDensity	If true, update coarse density for the block covered by this emitter
velocity	Default target velocity value.
velocityIsWorldSpace	If true, do not rotate velocity by xform
velocityScale	Multiplier for velocity and pointVelocities

Emitter Texture

Texture emitters allow simple arrays to define dense 3D grids of values.

Note

Mapping of Array Index to Texel: arrayIndex = (texelIndex.z * textureHeight + texelIndex.y) * textureWidth + texelIndex.x

textureName:assetPath and textureName:channel enable the use of standard image file formats as input to the texture emitter. Current channel values supported are red, green, blue, alpha, redSrgb, greenSrgb, blueSrgb, alphaSrgb, redSnorm, greenSnorm, blueSnorm, alphaSnorm. For Vec3 velocity, a red channel value will use red, green, blue to drive x, y, z.

Property	Result
allocationScale	1.0 allocates blocks needed to contain box defined by halfSize/position. 0.0 disables allocation. Values higher than 1.0 can be used to add additional padding.
applyPostPressure	If true, apply velocity after pressure solve. This is useful for collision, as it allows hard enforcement of velocity inside moving bodies.
burn	Default target burn value.
burnScale	Multiplier for burn and textureBurns
colorIsSrgb	If true, perform color correction on temperature, fuel, burn channels
coupleRateBurn	Default couple rate for burn channel.
coupleRateDivergence	Default couple rate for divergence channel.
coupleRateFuel	Default couple rate for fuel channel.
coupleRateSmoke	Default couple rate for smoke channel.
coupleRateRateTemperature	Default couple rate for temperature channel.
coupleRateVelocity	Default couple rate for velocity channel.
divergence	Default target divergence value.
divergenceScale	Multiplier for divergence and textureDivergences
enabled	Boolean to enable/disable emitter.
fuel	Default target fuel value.
fuelScale	Multiplier for fuel and textureFuels
halfSize	Half of box width/height/depth in world units.
layer	Layer integer. See Layer section.
position	World space position.
smoke	Default target smoke value.
smokeScale	Multiplier for smoke and textureSmokes
temperature	Default target temperature value.
temperatureScale	Multiplier for temperature and temperatureScale
textureBurns	Array of target burn values for each texel.
textureCoupleRateBurns	Array of burn couple rates for each texel.
textureCoupleRateDivergences	Array of divergence couple rates for each texel.
textureCoupleRateFuels	Array of fuel couple rate for each texel.
textureCoupleRateSmokes	Array of smoke couple rates for each texel.
textureCoupleRateTemperatures	Array of temperature couple rates for each texel.
textureCoupleRateVelocities	Array of velocity couple rates for each texel.
textureDepth	Depth in texels of texture data.
textureDivergences	Array of target divergence values for each texel.
textureFirstElement	Array index offset applied for all arrays. Useful for animation.
textureFuels	Array of target fuel values for each texel.
textureHeight	Height in texels of texture data.
textureSmokes	Array of target smoke values for each texel.
textureTemperatures	Array of target temperature values for each texel.
textureVelocities	Array of target velocity values for each texel.
textureWidth	Width in texels of texture data.
velocity	Default target velocity value.
velocityIsWorldSpace	If true, do not rotate velocity by xform
velocityScale	Multiplier for velocity and textureVelocities

Emitter NanoVDB

NanoVDB emitters allow sparse voxel data structures to define emitter shapes/values.

Property	Result
allocateActiveLeaves	If true, finer allocation granularity is used. (Adds some CPU overhead.)
allocationScale	1.0 allocates blocks needed to contain NanoVDB. 0.0 disables allocation. Values higher than 1.0 can be used to add additional padding.
applyPostPressure	If true, apply velocity after pressure solve. This is useful for collision, as it allows hard enforcement of velocity inside moving bodies.
burn	Default target burn value.
burnScale	Multiplier for burn and nanoVdbBurns.
colorIsSrgb	If true, perform color correction on temperature, fuel, burn channels
coupleRateBurn	Default couple rate for burn channel.
coupleRateDivergence	Default couple rate for divergence channel.
coupleRateFuel	Default couple rate for fuel channel.
coupleRateSmoke	Default couple rate for smoke channel.
coupleRateTemperature	Default couple rate for temperature channel.
coupleRateVelocity	Default couple rate for velocity channel.
divergence	Default target divergence value.
divergenceScale	Multiplier for divergence and nanoVdbDivergences
enabled	Boolean to enable/disable emitter
fuel	Default target fuel value.
fuelScale	Multiplier for fuel and nanoVdbFuels
layer	Layer integer. See Layer section.
maxDistance	Maximum distance for emitter active region. Used with nanoVdbDistances.
minDistance	Minimum distance for emitter active ergion. Used with nanoVdbDistances.
nanoVdbBurnFirstElement	Burn channel. Word offset applied when reading nanoVdbBurns array.
nanoVdbBurns	Burn channel. Word array containing NanoVDB data structure
nanoVdbBurns::assetPath	Burn channel. Optional path to OpenVDB/NanoVDB asset to use. Overrides nanoVdbBurns.
nanoVdbBurns::gridName	Burn channel. Grid name to search for in NanoVDB data structure.
nanoVdbCoupleRateBurnFirstElement	Burn couple rate channel. Word offset applied when reading nanoVdbCoupleRateBurns array.
nanoVdbCoupleRateBurns	Burn couple rate channel. Word array containing NanoVDB data structure
nanoVdbCoupleRateBurns::assetPath	Burn couple rate channel. Optional path to OpenVDB/NanoVDB asset to use. Overrides nanoVdbCoupleRateBurns.
nanoVdbCoupleRateBurns::gridName	Burn couple rate channel. Grid name to search for in NanoVDB data structure.
nanoVdbCoupleRateDivergenceFirstElement	Divergence couple rate channel. Word offset applied when reading nanoVdbCoupleRateDivergences array.
nanoVdbCoupleRateDivergences	Divergence couple rate channel. Word array containing NanoVDB data structure
nanoVdbCoupleRateDivergences::assetPath	Divergence couple rate channel. Optional path to OpenVDB/NanoVDB asset to use. Overrides nanoVdbCoupleRateDivergences.
nanoVdbCoupleRateDivergences::gridName	Divergence couple rate channel. Grid name to search for in NanoVDB data structure.
nanoVdbCoupleRateFuelFirstElement	Fuel couple rate channel. Word offset applied when reading nanoVdbCoupleRateFuels array.
nanoVdbCoupleRateFuels	Fuel couple rate channel. Word array containing NanoVDB data structure
nanoVdbCoupleRateFuels::assetPath	Fuel couple rate channel. Optional path to OpenVDB/NanoVDB asset to use. Overrides nanoVdbCoupleRateFuels.
nanoVdbCoupleRateFuels::gridName	Fuel couple rate channel. Grid name to search for in NanoVDB data structure.
nanoVdbCoupleRateSmokeFirstElement	Smoke couple rate channel. Word offset applied when reading nanoVdbCoupleRateSmokes array.
nanoVdbCoupleRateSmokes	Smoke couple rate channel. Word array containing NanoVDB data structure
nanoVdbCoupleRateSmokes::assetPath	Smoke couple rate channel. Optional path to OpenVDB/NanoVDB asset to use. Overrides nanoVdbCoupleRateSmokes.
nanoVdbCoupleRateSmoke::gridName	Smoke couple rate channel. Grid name to search for in NanoVDB data structure.
nanoVdbCoupleRateTemperatureFirstElement	Temperature couple rate channel. Word offset applied when reading nanoVdbCoupleRateTemperatures array.
nanoVdbCoupleRateTemperatures	Temperature couple rate channel. Word array containing NanoVDB data structure
nanoVdbCoupleRateTemperatures::assetPath	Temperature couple rate channel. Optional path to OpenVDB/NanoVDB asset to use. Overrides nanoVdbCoupleRateTemperatures.
nanoVdbCoupleRateTemperature::gridName	Temperature couple rate channel. Grid name to search for in NanoVDB data structure.
nanoVdbCoupleRateVelocities	Velocity couple rate channel. Word array containing NanoVDB data structure
nanoVdbCoupleRateVelocities::assetPath	Velocity couple rate channel. Optional path to OpenVDB/NanoVDB asset to use. Overrides nanoVdbCoupleRateVelocities.
nanoVdbCoupleRateVelocities::gridName	Velocity couple rate channel. Grid name to search for in NanoVDB data structure.
nanoVdbCoupleRateVelocityFirstElement	Velocity couple rate channel. Word offset applied when reading nanoVdbCoupleRateVelocities array.
nanoVdbDistanceFirstElement	Distance channel. Word offset applied when reading nanoVdbDistances array.
nanoVdbDistances	Distance channel. Word array containing NanoVDB data structure
nanoVdbDistances::assetPath	Distance channel. Optional path to OpenVDB/NanoVDB asset to use. Overrides nanoVdbDistances.
nanoVdbDistances::gridName	Distance channel. Grid name to search for in NanoVDB data structure.
nanoVdbDivergenceFirstElement	Divergence channel. Word offset applied when reading nanoVdbDivergences array.
nanoVdbDivergences	Divergence channel. Word array containing NanoVDB data structure
nanoVdbDivergences::assetPath	Divergence channel. Optional path to OpenVDB/NanoVDB asset to use. Overrides nanoVdbDivergences.
nanoVdbDivergences::gridName	Divergence channel. Grid name to search for in NanoVDB data structure.
nanoVdbFuelFirstElement	Fuel channel. Word offset applied when reading nanoVdbFuels array.
nanoVdbFuels	Fuel channel. Word array containing NanoVDB data structure
nanoVdbFuels::assetPath	Fuel channel. Optional path to OpenVDB/NanoVDB asset to use. Overrides nanoVdbFuels.
nanoVdbFuels::gridName	Fuel channel. Grid name to search for in NanoVDB data structure.
nanoVdbSmokeFirstElement	Smoke channel. Word offset applied when reading nanoVdbSmokes array.
nanoVdbSmokes	Smoke channel. Word array containing NanoVDB data structure
nanoVdbSmokes::assetPath	Smoke channel. Optional path to OpenVDB/NanoVDB asset to use. Overrides nanoVdbSmokes.
nanoVdbSmokes::gridName	Smoke channel. Grid name to search for in NanoVDB data structure.
nanoVdbTemperatureFirstElement	Temperature channel. Word offset applied when reading nanoVdbTemperatures array.
nanoVdbTemperatures	Temperature channel. Word array containing NanoVDB data structure
nanoVdbTemperatures::assetPath	Temperature channel. Optional path to OpenVDB/NanoVDB asset to use. Overrides nanoVdbTemperatures.
nanoVdbTemperatures::gridName	Temperature channel. Grid name to search for in NanoVDB data structure.
nanoVdbVelocities	Velocity channel. Word array containing NanoVDB data structure
nanoVdbVelocities::assetPath	Velocity channel. Optional path to OpenVDB/NanoVDB asset to use. Overrides nanoVdbVelocities.
nanoVdbVelocities::gridName	Velocity channel. Grid name to search for in NanoVDB data structure.
nanoVdbVelocityFirstElement	Velocity channel. Word offset applied when reading nanoVdbVelocities array.
smoke	Default target smoke value.
smokeScale	Multiplier for smoke and nanoVdbSmokes
temperature	Default target temperature value.
temperatureScale	Multiplier for temperature and nanoVdbTemperatures
velocity	Default target velocity value.
velocityIsWorldSpace	If true, do not rotate velocity by xform
velocityScale	Multiplier for velocity and nanoVdbVelocities